struct TreeNode<'a> {  
    value: &'a str,  
    left: Option<Box<TreeNode<'a>>>,  
    // TODO: 添加 right 字段  
}  

impl<'a> TreeNode<'a> {  
    fn new(value: &'a str) -> Self {  
        TreeNode { value, left: None, right: None }  
    }  
}  

#[test]  
fn test_tree_lifetime() {  
    let root;  
    {  
        let s = "hello".to_string();  
        root = TreeNode::new(&s); // 应编译失败  
    }  
}  